1. Field of the Invention
The present invention relates to emulsion based drilling and well treatment fluids and methods of using such fluids in the oil and gas industry with improved environmental compatibility. More particularly, the present invention relates to surfactant-free emulsions and their use in subterranean applications.
2. Brief Description of Relevant Art
Emulsions usually comprise two immiscible phases. The two immiscible phases include a continuous (or external) phase and a discontinuous (or internal) phase. The discontinuous phase comprises the secondary fluid that usually exists in droplets in the continuous phase. Two varieties of emulsions are oil-in-water and water-in-oil. Oil-in-water emulsions usually include a fluid at least partially immiscible in an oleaginous fluid (usually an aqueous-based fluid) as the continuous phase and an oleaginous fluid as the discontinuous phase. Water-in-oil emulsions are the opposite, having the oleaginous fluid as the continuous phase and a fluid at least partially immiscible in the oleaginous fluid (usually an aqueous-based fluid) as the discontinuous phase. Water-in-oil emulsions may also be referred to as invert emulsions.
Such emulsions have been used widely in oil and gas applications. For instance, emulsion based fluids are widely used in the oil and gas industry for drilling and other subterranean treatment applications, including various drilling, production, and completion operations. These drilling and well treatment fluids may also be referred to as muds. Invert emulsions may be used when oleaginous-based treatment fluids are expected to have superior performance characteristics when compared with water-based muds, as in situations, e.g., where there is an abundance of water reactive materials in a well bore. These superior performance characteristics may include, e.g., better lubrication of the drill strings and downhole tools, thinner filter cake formation, and better hole stability. An emulsification of water-in-oil, without having any emulsifying agent capable of stabilizing the fluid that is at least partially immiscible in the oleaginous fluid typically will undergo a rapid and natural degradation processes including droplet coalescence and Ostwald ripening, until the two phases which are at least partially immiscible separate and the emulsion no longer exists. Having an unstable invert emulsion may be problematic because if the emulsion destabilizes, it may not have consistent, reliable properties. This problem may be exacerbated by the physical forces that the emulsion may undergo when being used in subterranean applications, such as thermal, mechanical, and chemical stresses. Emulsion stabilizing agents, sometimes referred to as emulsifiers, may be useful in invert emulsions (and emulsion based drilling and well treatment fluids) to stabilize the emulsions, especially when used in subterranean applications.
Emulsion stabilizing agents traditionally used in drilling and well treatment fluids are surfactant-based. Structurally, surfactant-based emulsion stabilizing agents usually comprise a hydrophobic portion—a tail—that is attracted to the oil phase and a hydrophilic portion—a head—that is attracted to the water phase. Generally, the hydrophobic portion interacts with the oil and the hydrophilic portion interacts with the nonoleaginous fluid. These interactions generally decrease the surface tension of the interface between the water droplet and the oil, which may slow the natural tendency of the two immiscible phases to separate.
However, surfactant-based emulsion stabilizing agents may be problematic, as they may suffer from problems that include some potential or possible toxicity, limited range of oil to water ratios, thermal destabilization, propensity for droplet coalescence, and intolerance to various salts and other chemical agents. The potential or possible toxicity of the surfactants can create potential dangers for the environment. For example, surfactants may have adverse effects on shrimp and other aqueous species, along with poor biodegradability. Further, a surfactant is typically capable of stabilizing either an oil-in-water emulsion or a water-in-oil emulsion, but not both. Because of the necessity to carefully balance the chemical interactions of the surfactant to the type of micelle formed, typical surfactants generally can be used only with a limited oil to water ratio range. Because they diffuse in and out of the micelles, surfactant-based agents typically form a meta-stable structure around the micelle. This meta-stable structure allows such forces as coalescence, which may result in phase separation and eventual emulsion instability.